Subgrade Construction Methodology & Quality Control
As per MoRTH Clause 305

Subgrade is the most critical structural layer of highway pavement. This guide provides a complete and practical methodology for subgrade construction as per MoRTH Clause 305 (5th Revision), including field quality control procedures, tolerances, inspection checklists, and acceptance criteria.

1. Scope of Subgrade Construction

1. Scope of Work

The work shall consist of construction of subgrade using approved and specified materials obtained from approved borrow areas or suitable material derived from roadway excavation and drain excavation, strictly in accordance with MoRTH Specifications Clause 305.

All operations shall be executed under controlled conditions ensuring compliance with approved drawings, quality assurance plan (QAP), and project specifications.

2. Reference Standards

• MoRTH Specifications (5th Revision) – Clause 305 & Clause 902
• IRC:37 – Guidelines for Design of Flexible Pavements
• IS 2720 – Methods of Test for Soils (Relevant Parts)
• Approved Contract Drawings & Project Specifications

3. Setting Out

After completion of site clearance and embankment formation, the limits of the subgrade shall be accurately marked using survey pegs at regular intervals on both sides.

A permanent working benchmark shall be established outside the construction zone to ensure uninterrupted reference for level control during execution.

Engineering Control Note: No earthwork shall commence without verified setting-out approval from the site survey and QA/QC engineer.

4. Material Requirements

1. General Requirement:
   The subgrade material shall consist of soil, moorum, gravel, reclaimed pavement material, fly ash, pond ash, or approved mixtures thereof, subject to compliance with MoRTH Clause 305.2.
   
   The material shall be free from organic matter such as roots, stumps, rubbish, or any deleterious substances that may affect the strength, stability, or durability of the subgrade.
   
   Preference shall be given to usable material obtained from roadway excavation or other approved excavation sources within the project limits.



5. Borrow Material – Investigation, Testing & Approval

Borrow areas shall be systematically investigated by gridding the area at approximately 25 m c/c, or closer spacing where variability in soil strata is observed, extending to the full depth of proposed excavation.

Test pits shall be excavated, properly logged, and plotted to establish stratification and identify zones of suitable and unsuitable materials. This shall form the basis for selection and approval of borrow sources by the Engineer.

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5.1 Borrow Area Sampling & Testing Requirements



6. Equipment, Tools & Manpower Deployment (Subgrade Works)

All equipment, tools, and manpower deployed for subgrade construction shall be adequate in capacity, well-maintained, and approved prior to commencement of work. The Contractor shall ensure that all resources are available at site and demonstrate the operational efficiency of plant and machinery whenever required by the Engineer. Equipment requiring calibration or certification shall possess valid calibration records.

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6.1 Equipment Deployment

The following plant and machinery shall be deployed for the systematic execution of subgrade works:

Equipment	Purpose
Hydraulic Excavator / Backhoe Loader	Excavation and material handling
Dozer	Spreading and rough grading
Motor Grader	Final grading and profile correction
Dumpers / Tipper Trucks	Transportation of approved earth material
Vibratory Roller (Minimum 8–10 T or as specified)	Layer-wise compaction
Water Tanker with Sprinkler	Moisture conditioning and dust suppression
Tractor Trolley (if required)	Auxiliary transportation
Auto Level / Total Station	Survey, line, level and cross-section control
Measuring Tape, Staff & Survey Instruments	Dimensional verification

Engineering Control: All equipment shall be maintained in sound working condition. Compaction equipment shall be suitable for the specified layer thickness and soil type to achieve the required density as per MoRTH Specifications.

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6.2 Tools & Accessories

• Hand Hammer
• Crow Bar
• Spade / Shovel
• Pick Axe
• String Line
• Survey Pegs / Wooden Stakes
• Lime Powder for Marking
• Measuring Tape
• Spirit Level (where required)
• Safety Barricades & Traffic Cones

Site Requirement: Adequate quantities of tools shall be available at site to facilitate continuous construction activities without interruption.

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6.3 Manpower Deployment & Responsibilities

Qualified personnel shall be deployed throughout the execution of subgrade works to ensure compliance with project specifications, quality requirements, and safety regulations.

• Construction Manager
  Overall responsible for planning, execution, resource mobilization, coordination with the Engineer, monitoring construction progress, quality assurance, safety compliance, and adherence to approved method statements.


• Site Engineer
  Responsible for day-to-day supervision of construction activities, ensuring compliance with approved drawings, MoRTH Specifications, quality requirements, and inspection readiness.


• Land Surveyor
  Responsible for setting out, establishment of benchmarks, verification of alignment, levels, cross fall, finished formation, and maintenance of survey records.


• Material Engineer / QA-QC Engineer
  Responsible for approval of materials, laboratory and field testing, quality documentation, verification of moisture content, field density, and compliance with project specifications.


• Safety Supervisor
  Responsible for implementation of the Safety Management Plan, conducting toolbox talks, ensuring PPE compliance, hazard identification, traffic management, and statutory safety requirements.


• Plant & Equipment Operators
  Responsible for safe and efficient operation of construction equipment, daily equipment inspection, preventive maintenance checks, and reporting of defects.


• Skilled & Unskilled Labour
  Responsible for spreading, leveling, trimming, moisture conditioning, compaction support, housekeeping, and assisting construction activities under the supervision of the Site Engineer.

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6.4 QA/QC Requirements

No subgrade construction activity shall commence unless the following requirements have been fulfilled:

• Approved borrow material is available at site.
• Survey setting-out has been completed and verified.
• Plant and machinery have been inspected and are in satisfactory working condition.
• Required manpower has been deployed.
• Inspection Requests (IRs), wherever applicable, have been approved.
• Field laboratory facilities and testing equipment are ready for quality control activities.
• Safety clearances and work permits have been obtained, wherever applicable.

Quality Control Note: The Engineer reserves the right to suspend the work if the deployed resources, equipment, or manpower are found inadequate to achieve the specified quality requirements under the Contract and MoRTH Specifications.

5. Construction Methodology

1. After completion of embankment, the limits of subgrade shall be marked by fixing pegs at regular intervals before commencing the earthwork. Construction toe line will be more than the design toe line to ensure proper compaction on edges.
2. The embankment top shall be levelled to facilitate placement of first layer of subgrade, scarified, mixed with water and then compacted by rolling so as to achieve minimum dry density as specified in Table 300-1 of MORTH specifications. Subgrade treatment specified for subgrade shall be carried out as per approved design and drawing. Unsuitable material occurring in subgrade foundation shall be removed as per direction and approval of AE and replaced by approved materials laid in layers and suitably compacted to the required and specified degree of compaction. Excavation at approved borrow areas shall be carried out with excavator and transportation of excavated material shall be done by dumpers.
3. M.D.D. of borrowarea soil as determined in lab, and when the same be laid for subgrade preparation and tested for dry density should not have relative compaction less than 97% of M.D.D. (as specified in MORTH specifications Table 300-2), if found less, the layer shall be scarified mixed with water and compacted to achieve minimum specified dry density.
4. The subgrade material shall then be spread in layers not exceeding 250 mm (when compacted using vibratory roller / compactor) or up to 200mm (when compacted using 80-100 kN static roller) thicknesses over the entire area with the dozer and finished by a motor grader. When the existing subgrade to be widened continuous horizontal benches shall be cut out into the old slope to ensure adequate bond. Where water is required to be added the same shall be sprinkled from a water tanker fitted with sprinkler, uniformly on the surface but without flooding and shall be mixed thoroughly in soil by blending or harrowing until uniform moisture content is obtained throughout the depth of the layer. If the material delivered to the roadbed is too wet, it shall be dried by aeration and exposure to the sun till the moisture content is accepted for compaction.
5. The compaction shall be done with the help of vibratory roller or 8 - 10 tonne static roller. Compaction trial shall be carried out on a suitable stretch to determine the no. of passes required for particular type of soil, dumped for subgrade, to achieve densities specified in the Table 300-2 of MORTH specifications. Based on the trial actual compaction will be carried out with required no. of passes of vibratory roller or static roller until the specified density is achieved. Rolling shall progress parallel to the centreline of the road uniformly overlapping each preceding track by one-third width. Rolling shall be continued till the specified density is achieved as per Table 300-2 of MORTH specifications. In case specific compaction is not achieved the material in the soft area shall be removed and replaced with approved material. Densities shall be tested by sand replacement method.
6. Subsequent layers shall be placed only after the finished layer has been tested as per the clause 903.2.2 and accepted. The top levels of the subgrade shall be checked with reference to the longitudinal and cross profile of the road as per drawing to keep the variation within tolerance limit as per table 900-1.
7. The Safety Officer shall make frequent patrols along the highway to ensure that the safety equipment and signs are operational at all time.

6. Tests & Frequency

Test to be conducted for Subgrade

Test / Parameter	Standard	Frequency	Acceptance Criteria
Grain Size Analysis (GSA)	IS: 2720 (Part 4)	2 Tests / 3000 m³	50 mm below material to be used
Atterberg Limits	IS: 2720 (Part 5)	2 Tests / 3000 m³	LL ≤ 50%
Plasticity Index	IS: 2720 (Part 5)	2 Tests / 3000 m³	PI ≤ 25%
Free Swelling Index	IS: 2720 (Part 40)	As required	≤ 50%
Modified Proctor	IS: 2720 (Part 8)	2 Tests / 3000 m³	MDD ≥ 1.75 g/cc
Deleterious Content	IS: 2720 (Part 27)	As required	(a) Sodium Sulphate ≤ 0.5% (b) Organic Matter ≤ 1.0%
CBR (4 Days Soaked)	IS: 2720 (Part 16)	1 Test / 3000 m³	As per Design

Activity	Standard	Frequency	Acceptance Criteria
In-process Inspection
Alignment, Cross-section & Level	Standard Survey Practice	Continuous	As per design
Moisture Content	IS: 2720 (Part 2)	Each & every pit	-2% to +1% of OMC
Field Density (Sand Replacement)	IS: 2720 (Part 28)	0–2000 m² = 1 set (10 observations)	≥ 97%
Field Density (Nuclear Gauge)	AASHTO T310	0–2000 m² = 2 sets (10 observations each)	≥ 97%

7. Tolerances & Acceptance Criteria

Parameter	Permissible Tolerance
Compaction	≥ 97% MDD
Moisture Content	±2% of OMC
Finished Level	±20 mm
Width	+100 mm / -50 mm
Cross Slope	±0.3%

8. Quality Control Checklist

The following quality control checklist shall be followed during and after subgrade construction to ensure compliance with MoRTH Specifications, approved drawings, and project quality requirements.

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8.1 During Construction

• Verify approved borrow material is being used.
• Confirm survey setting-out, line, level, and formation width.
• Check layer thickness before compaction.
• Ensure moisture content is maintained within Optimum Moisture Content (OMC) ±2% or as specified.
• Confirm uniform spreading and proper grading of material.
• Ensure specified rolling pattern and adequate roller passes are followed.
• Prevent segregation of soil during transportation and spreading.
• Monitor field density tests at the specified frequency.
• Inspect cross fall, camber, and drainage during construction.
• Ensure no traffic movement over uncompacted layers.
• Verify compliance with approved Method Statement and ITP.
• Ensure safety measures and PPE compliance throughout the activity.

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8.2 After Construction

• Required field density achieved as per MoRTH Specifications.
• Finished levels, alignment, width, and cross slope verified.
• Surface free from loose material, rutting, and soft patches.
• Surface tolerance checked using a straight edge where applicable.
• Field moisture and density test reports reviewed and accepted.
• Inspection Request (IR) approved by the Engineer.
• All quality control records completed and filed.
• Subgrade approved before placement of the subsequent pavement layer.

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QA/QC Requirement: No subsequent pavement layer shall be laid until the completed subgrade has been inspected, tested, and formally approved by the Engineer. Any non-conforming area shall be rectified and re-tested before proceeding with further construction activities.

9. Common Defects, Causes & Rectification

During subgrade construction, defects may arise due to improper material selection, inadequate moisture control, poor compaction practices, or non-compliance with construction methodology. The following table summarizes the common defects, their likely causes, and recommended corrective measures.

Defect	Possible Cause	Recommended Rectification
Low Field Density	Insufficient rolling, incorrect moisture content, excessive layer thickness	Scarify the layer, adjust moisture to OMC, relay and recompact until the specified density is achieved.
Over-Wet Layer	Excessive water application or rainfall	Aerate the material, allow drying to near OMC, remix thoroughly and recompact.
Dry Layer	Moisture content below Optimum Moisture Content (OMC)	Apply water uniformly using a water tanker, remix the material and compact immediately.
Uneven Surface	Improper grading or inadequate finishing	Trim using a motor grader, correct line and level, and recompact the affected area.
Segregation of Material	Improper dumping or spreading of soil	Remove segregated material, remix thoroughly, spread uniformly and recompact.
Surface Rutting	Weak foundation, excessive moisture or premature traffic movement	Remove affected material, reconstruct the layer and restrict traffic until approval.
Ponding of Water	Improper cross fall or inadequate drainage	Re-establish the required cross slope, improve drainage and compact the corrected surface.
Cracks on Surface	Drying shrinkage or inadequate moisture control	Scarify the cracked portion, adjust moisture, remix and recompact.

QA/QC Requirement: Any portion of the subgrade failing to meet the specified density, moisture content, line, level, thickness, or surface tolerance shall be rejected and rectified before subsequent pavement layers are placed. Corrective measures shall be carried out at the Contractor's own cost and shall be subject to re-inspection and approval by the Engineer.

10. Frequently Asked Questions (FAQs)

The following frequently asked questions address common site queries related to subgrade construction in accordance with MoRTH Specifications for Road and Bridge Works.

S.No.	Question	Answer
1	What is the minimum compaction requirement for subgrade?	Subgrade shall be compacted to a minimum of 97% of the Maximum Dry Density (MDD) determined as per IS:2720 (Part 8), unless otherwise specified in the Contract or MoRTH Specifications.
2	What is the maximum compacted layer thickness?	The compacted thickness shall generally not exceed 250 mm with a vibratory roller and 200 mm with a static roller, unless otherwise approved by the Engineer.
3	What should be the moisture content during compaction?	Compaction shall be carried out at or near the Optimum Moisture Content (OMC), generally within +1% and −2% of OMC or as specified in the project specifications.
4	How frequently should Field Density Tests (FDT) be conducted?	Field Density Tests shall be conducted at the frequency specified in the Contract or MoRTH Specifications. A common requirement is 10 tests per 2,000 m² of completed subgrade.
5	What should be done if the required density is not achieved?	The layer shall be scarified, moisture content adjusted to OMC, reworked, and recompacted. If satisfactory compaction is still not achieved, the unsuitable material shall be removed and replaced with approved material.
6	What materials are suitable for subgrade construction?	Approved soil, moorum, gravel, selected earth, fly ash, pond ash, or other approved materials meeting the requirements of the project specifications and free from organic matter and deleterious substances may be used.
7	Can construction proceed during rainfall?	No. Earthwork and compaction operations should be suspended during rainfall or when the material becomes excessively wet. Construction may resume only after the moisture content is brought back within the specified limits.
8	Which field tests are normally carried out for subgrade quality control?	Common quality control tests include Moisture Content, Field Density Test (Sand Replacement or Nuclear Gauge), CBR (where applicable), Plate Load Test (if specified), and survey checks for line, level, thickness, and cross slope.
9	What are the common causes of subgrade failure?	Poor-quality material, inadequate compaction, improper moisture control, insufficient drainage, excessive layer thickness, and premature traffic movement are the most common causes of failure.
10	Can the next pavement layer be laid immediately after subgrade construction?	No. The completed subgrade shall be inspected, tested, and approved by the Engineer. The subsequent pavement layer shall only be laid after all quality requirements have been satisfactorily met.